Organo-modified silicone polymers are used in multitude of applications such as healthcare, personal care, home care, coatings, agricultural compositions etc. The presence of enough organic content can bring significant changes in properties associated with bare silicones. The common approach to synthesizing silicone-organic polymer is bulk polymerization of silicone monomers/macromers with organic monomers/macromers. This leads to randomized structure with an uncontrolled degree of cross-linking which hampers reproducibility in synthesis and thereby the final properties. There is a need to develop an approach to carefully design and build silicone-organic polymer with well-defined structure, controlled composition and tunable structure-property relationship. These polymers can have terminal or pendant reactive groups for further polymerization i.e. pre-polymer, which make them useful in reactive composition such as copolymers, hydrogels, coating, emulsions/latex etc.
Curable silicone-hydrogel formulations are used to make extended wear soft contact lenses due to their relatively high oxygen permeability, flexibility, comfort, and reduced corneal complications. Conventional hydrogel materials (e.g. 2-hydroxyethyl methacrylate, HEMA) by themselves have poor oxygen permeability and they transport oxygen to the eye through the absorbed water molecules. Water has low oxygen permeability, also called the Dk value, which may be expressed in Barrer, wherein 1 Barrer=10−11 (cm3 O2) cm cm−2 s−1 mmHg−1 where “cm3 O2” is at a quantity of oxygen at standard temperature and pressure and where “cm” represents the thickness of the material and “cm−2” is the reciprocal of the surface area of that material. The Dk of water is 80 Barrer. Upon exposure to atmospheric air for long periods, these lenses are slowly dehydrated and the amount of oxygen transported to the cornea is reduced. Eye irritation, redness and other corneal complications can result and hence restrict use of the lenses to limited periods of wear.
Silicone-hydrogel materials and films for contact lenses are popular for their high oxygen permeability, flexibility and comfort. Silicone material, however have poor wettability, and hence several methods have been developed to improve the water content of these hydrogel formulations. This includes adding hydrophilic monomers to the hydrogel formulations. But this causes incompatibility between silicone and organic monomers and leads to phase separation.
A possible solution to this problem is to make the silicone monomer inherently hydrophilic by incorporating hydrophilic units in the monomer. One approach to provide hydrophilic silicone monomers is to polymerize the organo-modified silicone monomer with organic monomers in the presence of a cross-linker. Examples of prior attempts of providing hydrophilicity include those described in U.S. Pat. Nos. 4,260,725; 5,352,714; 5,998,498; 6,867,245; 6,013,711; and 6,207,782. This approach leads to a large number of unreacted monomers due to unregulated viscosity build-up that requires extracting the leachable monomers from the matrix by water-isopropanol solvent mixtures, which leads to increased processing costs. Further, the silicone hydrogel formulations made by these methods still fail to exhibit significant wettability. To overcome this, more hydrophilic monomers or internal wetting agents are added to the hydrogel compositions, but this compromises oxygen permeability. Alternatively, a secondary treatment such as “plasma oxidation” can be used, but this treatment is expensive.
The use of pre-polymers is one approach to providing silicone monomers with improved hydrophilicity and oxygen permeability that can be cured in a controlled fashion so as to reduce leachable monomers/oligomers, processing cost and toxicity. The pre-polymer approach ties up the silicone chemistry with polymerization techniques to synthesize silicone-organic polymers with a well-defined structure and controlled composition. Significantly high hydrophilicity can be achieved without compromising oxygen permeability. Further, the polymer composition can be tunable such that it can be tailored to provide particular properties depending on the intended use. These polymers are further functionalized with a reactive group and introduced into a curable composition. This concept brings in reproducibility and increases the purity of the final materials.
Some prior attempts to provide suitable pre-polymers include the approach of U.S. Pat. No. 7,268,189, which describes polysiloxane based cross-linkers used in combination with a hydrophilic monomer in the presence of a chain transfer agent to yield an amphiphilic pre-polymer. U.S. Pat. No. 5,981,669 relates to the synthesis of a mono-functional pre-polymer by the free-radical polymerization of a silicone monomer and a hydrophilic monomer in the presence of a chain transfer agent. These pre-polymers were then introduced into formulations with bi-functional macromer which may be composed of silicone. U.S. Patent Publication Nos. 2011/0166248A1 and 2008/0231798 describe block copolymers of silicone-containing monomers and hydrophilic monomers to yield a pre-polymer. U.S. Patent Publication No. 2010/0298446 reports functionalization of polysiloxane blocks to act as a macro initiator for polymerizing a hydrophilic monomer via. atom transfer radical polymerization (ATRP). This technique yields bi- or tetra-functional pre-polymer. U.S. Patent Publication No. 2010/0296049 describes a reversible addition fragmentation chain transfer (RAFT) technique for polymerizing a mixture of a bi-functional polysiloxane polymer and an organic monomer. U.S. Patent Publication No. 2009/0143499 describes, a pre-polymer made of polysiloxane blocks, poly(oxyalkylene) blocks, and cross-linkable groups.
Conventionally, silicone-hydrogels are made by polymerizing the acrylate or methacrylate functionalized silicone monomer with hydrogel (hydrophilic) monomers, such as 2-hydroxyethyl methacrylate (HEMA), N-vinylpyrrolidone (NVP) and other monomers such as methyl methacrylic acid (MMA), N,N-dimethylacrylamide (DMA), etc., in the presence of cross-linker and free radical or photoinitiators. Cross-linking agents generally have two or more reactive functional groups at different sites of the molecule. Typically, these sites contain polymerizable ethylenic unsaturation groups. During curing, they form a covalent bond with two different polymer chains and form a stable three-dimensional network to improve the strength of the polymer. Cross-linking agents conventionally used in contact lenses include ethylene glycol dimethacrylate and trimethyloylpropane trimethacrylate (about 0.1 to 2 wt. %). Other useful cross-linking agents include diethyleneglycol dimethacrylate, bisphenol A dimethacrylate, diglycidyl bisphenol A dimethacrylate and dimethacrylate-terminated polyethylene glycol and reactive linear polyether modified silicones.
Generally, silicone hydrogel contact lens materials are made using either hydrophobic mono-functional silicone monomer (such as TRIS) or multi-functional hydrophilic silicone monomer followed by secondary surface treatment. Mono-functional silicone monomers are preferred in the contact lens industry over multi-functional silicone monomers since the latter lead to increased rigidity of the lens made therefrom.
The state of this art for soft contact lenses, including the silicone-based materials described in the above mentioned patents, still possess major shortfalls like sub-optimal surface wettability and lipid deposition. There remains a need for hydrophilic silicone monomers with advantageous wettability and oxygen permeability that can be used to make contact lenses without the drawbacks and expensive surface treatments necessary with the silicone containing materials of the current state of art.